Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Sun Apr 21 2024 Valid 12Z Wed Apr 24 2024 - 12Z Sun Apr 28 2024 ***Showers and thunderstorms with heavy rainfall increase in coverage across the Central U.S. by the end of the week*** ...General Overview... The forecast period begins on Wednesday with an amplifying upper trough over the Northeast U.S., and this will propel a cold front across much of the Eastern U.S. for the middle of the week. Surface high pressure builds in across the Great Lakes and then across the East Coast region to close out the week. Meanwhile, a synoptic scale trough develops across the Western U.S. by Friday and into next weekend with a couple of shortwaves ejecting out across the Plains, and supporting surface cyclogenesis east of the Rockies. This will likely result in multiple rounds of showers and storms for the Central U.S., with some heavy rain and severe weather likely by the end of the week. ...Guidance/Predictability Assessment... The first forecast challenge is centered on the approach of two shortwaves in the northern stream, potentially phasing and closing off in the Northeast by the Wednesday-Thursday time period. The GFS and its ensemble mean continue to support a stronger/more phased system that features a deep closed low over northern New York by 00Z Thursday. The 00Z non-NCEP guidance has trended more amplified compared to the 12Z runs, but have the core of the closed low farther north than the GFS. The other area of concern is across the West with the next round of shortwave energy moving into the Desert Southwest and eventually ejecting out across the Plains by Friday. Recent runs of the GFS have been faster and somewhat stronger compared to the non-NCEP guidance. Then the next system behind that amplifies further across the Western U.S. over the weekend, with relatively good agreement among the ECMWF and GFS, although recent CMC runs have varied more with the depth and timing of the trough. The fronts/pressures forecast was primarily derived from a general deterministic model blend early on, with slightly more weighting to the ECMWF and CMC through Thursday, and then gradually increasing proportions of the ensemble means to about 50% by Sunday. ...Weather/Hazards Highlights... The upper trough and cold front passing through the Northeast U.S. Wednesday will be accompanied by widespread light to moderate rain, and high elevation snow for portions of New Hampshire and Maine. The GFS is the heaviest solution for QPF given its stronger upper level system. Farther west, warm air advection across Texas and Oklahoma will bring a return to scattered showers and storms across this region, with some of the model guidance suggesting the potential for some local one inch rainfall totals. However, the guidance is not locked in on the location of these scattered QPF maxima, and therefore it seems prudent to hold off on any potential risk areas for the Day 4 outlook on Wednesday until a better model signal develops for placement. More widespread showers and storms are then expected to develop across portions of Kansas, Nebraska, Iowa, and Missouri going into Thursday (Day 5) as moisture and elevated instability increase with surface low pressure system development across the Central Plains. There is enough support in recent model runs to support a Marginal Risk area where multiple thunderstorm complexes are likely to develop. The potential also exists for some strong to severe storms ahead of the dryline in the Southern Plains and extending northeast to Arkansas to close out the work week, so this will continue to be monitored and the Storm Prediction Center has additional information pertaining to this. Much of the West should have above normal temperatures on Wednesday, before returning to near or slightly below normal levels for much of the West Coast and Intermountain West to close out the week, as the upper level trough encompasses the region. Warmer temperatures and increasing humidity are forecast for the central/southern Plains by next Friday and beyond as the upper ridge axis builds back, and southerly flow from the western Gulf advects moisture northward ahead of the developing surface low. This spreads northeastward to include the Ohio Valley and the Appalachians, with more widespread above normal readings for overnight lows (+10 to +20 degrees) with conditions more typical of May or early June by next weekend. Highs could get over 100 degrees for portions of southern Texas by Saturday. Hamrick Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw